Turbochargers are a type of forced induction system. They compress the air flowing into an engine, thus boosting the engine's horsepower without significantly increasing weight. Turbochargers use the exhaust flow from the engine to spin a turbine, which in turn drives an air compressor. Since the turbine spins about 30 times faster than most car engines and it is hooked up to the exhaust, the temperature in the turbine is very high. Additionally, due to the resulting high velocity of flow, turbochargers are subjected to noise and vibration. Such conditions can have a detrimental effect on the components of the turbocharger, particularly on the rotating parts such as the turbine rotor, which can lead to failure of the system.
The pressure developed by a centrifugal type of turbocharger however, varies as the square of the engine speed. An increase in delivery pressure sufficient to provide the required increased torque at low engine speed will greatly increase the delivery pressure at higher engine speed.
This problem has been overcome by incorporating a wastegate valve into the turbine side of the turbocharger. The wastegate valve allows engine exhaust gas to bypass the turbine nozzle and wheel as required. This serves to limit turbocharger speed at high engine speeds and loads. In doing so, the wastegate valve reduces the boost pressure attained at full speed full load to a desired amount.
One such contemporary wastegate valve assembly utilizes turbine pressure as the actuating mechanism. Such assemblies include a spring biased poppet valve arranged to open against the spring bias when the gas pressure entering the turbine wheel or the engine boost pressure exceeds a certain level. Opening of the poppet valve allows part of the exhaust gases to pass through the valve and to bypass the turbine nozzle and wheel. These valve assemblies have been shown to be unreliable and have a propensity for seizing in the closed position.
Another contemporary wastegate valve assembly is a controlled assembly that utilizes a pneumatic actuator to open and close the wastegate. These wastegate turbochargers are matched to give good performance at low engine speed with the valve closed. This can improve transient response and reduce exhaust temperatures and emissions. As engine speed increases, the wastegate valve begins to open at a pre-set boost pressure. This has the effect of increasing the swallowing capacity of the turbine, reducing shaft power and avoiding excess air delivery and rotor overspeed.
A turbocharger with a contemporary wastegate assembly is described in U.S. Pat. No. 6,658,846 to McEwan. The McEwan turbocharger 10, as shown in FIGS. 1 and 2, has a wastegate assembly 20 with a circular valve 30 positioned in the turbine section 40 for exhaust gas bypassing. The valve 30 is opened and closed by linkage 50 which is connected to a pneumatic control device 60. The pneumatic control device 60 is bolted to the compressor housing 70 by bolts 75 and is fed pressurized air from conduit 65. After assembly and mounting to the turbine and compressor housings 45, 70, the wastegate assembly 20 must then be calibrated in situ to ensure sufficient bypass at high engine speeds.
The McEwan wastegate assembly 20 suffers from several drawbacks. Assembly time and cost are increased because of the in situ calibration technique required, as well as the mounting across multiple sections of the turbocharger. The linkage 50 extends along a substantial length of the turbocharger 10 increasing the envelope of the turbocharger, which can be especially significant where space in an engine compartment is limited. Due to the limited access to the wastegate valve 30 once installed, machining to promote proper sealing is limited.
Thus, there is a need for a wastegate assembly that facilitates assembly. There is a further need for such an assembly that allows for pre-assembly and/or pre-calibration. There is yet a further need for such an assembly that facilitates machining for proper sealing. There is also a need for such an assembly that is cost effective and dependable.